Conventionally, fastening of a number of fastening members to a mounting base generally requires work to make the fastening members equal in clamping force so as to ensure required effective fastening strength. However, it has been difficult for any skilled worker to equalize the tightening torques to be applied to the fastening members even by using an automatic clamping device controlled numerically. Hence, adjustments have been made while checking the fixed state of the fastening member after fitting the fastening member to the mounting base in the existing circumstances.
Besides, the fastening member fitted to the mounting base gets inconveniently loose due to change in subsequent mounting conditions (deterioration, fatigue, desiccation, etc.) of the fastening member and the mounting base.
Under the circumstances, a need has been felt for a high-performance fastener capable of clamping numbers of fastening members with an equal clamping force and preventing slack of the fastening member for a long period of time.
There has been heretofore known a fastener to fulfill the need as disclosed in International Patent Publication No. WO99/40331.
The conventional fastener is fitted to a fastening member tightly screwed onto a mounting base while applying continually a tightening torque to the fastening member. The conventional fastener comprises a spiral spring turbinated in the contracted state to accumulate the tightening torque, which has one end serving as an engaging end in engagement with the fastening member and the other end serving as a fixing end fixed onto the mounting base, and a detachable stopper fitted to the torsion coil spring, which torsion coil spring releases the tightening torque accumulated thereby when removing the stopper to apply the tightening torque to the fastening member.
The conventional fastener is used by first engaging the engaging end of the spiral spring with the fastening member fixed onto the mounting base, securing the fixing end of the spiral spring to the mounting base, and then, letting the stopper off the spiral spring, consequently to exert a tightening torque produced by the elasticity of the spiral spring to the fastening member. As a result, the tightening of the fastening member is automatically regulated to prevent slack of the fastening member.
However, the conventional fastener has a disadvantage of being inevitably made large in overall size because it employs the spiral spring which expands or contracts radially during tightening operations. Besides, since the tightening torque produced by the spiral spring is dispersed in the radial direction, the torque to be applied to the fastening member is fatally weakened.